This application claims priority from Japanese patent application 2005-313968, filed Oct. 28, 2005. The entire content of the aforementioned application is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a displacement sensor including a projector, a photoreceiver including an aperture, and a lens unit including a lens reciprocatable along an optical axis of a coaxial optical system (called a coaxial confocal optical system) adjusted so that a light emission position of the projector and the aperture have a conjugate relation. The displacement sensor measures a displacement of an object to be measured on the basis of the position of the lens when a light reception amount signal of the photoreceiver displays a maximum value.
2. Description of the Related Art
A displacement sensor of this kind uses the principle that when light emitted from a projector via a lens unit condenses in a predetermined position, light reflected at the condense point travels in a path opposite to the projection path and condenses in the position of an aperture having a conjugate relation with the light emission position. Concretely, the displacement sensor emits a measurement beam that condenses in a predetermined position while reciprocating a predetermined lens in the lens unit along the optical axis, receives reflection light of the beam, and obtains a displacement of an object to be measured (hereinbelow, called a work) on the basis of the position of the lens when the light reception amount signal of the photoreceiver has the maximum value.
An example of the document disclosing the optical system and the principle of measuring process is Japanese Patent Application Laid-Open No. Hei 7-113617.
FIG. 8 shows the configuration of an optical system of a displacement sensor disclosed in Japanese Patent Application Laid-open No. Hei 7-113617. The optical system includes a projector 200 having a laser diode 201, a photoreceiver 204 including a photodiode 202 and a pin hole 203, a beam splitter 205, and a pair of lenses 206 and 207. In the lenses, the lens 206 closer to the projector and the photoreceiver is a collimate lens, and the other lens 207 is an objective lens.
The objective lens 207 is attached to the tip of a not-shown tuning fork and reciprocates along the optical axis in accordance with vibration of the tuning fork. When the laser diode 201 is allowed to emit light in this state, a beam condensing position of a measurement beam BM passed through the lenses 206 and 207 also changes. Therefore, when the beam condensing position of the measurement beam BM and the position of the surface of a work W coincide, reflection light from the work W converges at the pin hole 203, so that the light reception amount signal of the photodiode 202 increases. On the other hand, the phenomenon does not occur in the other cases, so that light is hardly incident on the photodiode 202. Therefore, based on the position of the objective lens 207 when the light reception amount signal has the maximum value, the beam condensing position of the measurement beam BM at the time point is obtained, and is set as the position of the work W.
Since the displacement sensor of this kind is used for applications of measuring a small displacement on a work such as an electrode pattern on a glass substrate, the optical system has to be adjusted so that a condensing range of a beam in the optical axis direction is limited to an extremely narrow range (in other words, the depth of focus becomes shallow).
On the other hand, the height of a reference face of the work and the magnitude of the displacement fluctuate according to the kinds of works. Consequently, there is a demand for freely changing a working distance of a sensor (the minimum distance between the light emission face of the sensor and the measurement beam condensing position) and a measuring range (range in which the condensing position of the optical beam moves).
One of methods addressing the demand is replacement of the objective lens. However, when the weight and diameter of the lens is changed, resonance frequency also changes. Consequently, the lens driving system has to be re-designed. Therefore, the demand is not addressed only by replacing the lens, and it is difficult to carry out the method.
As described in Japanese Patent Application Laid-Open No. 2004-102228, there is another method of disposing a divergent lens between an objective lens and a collimate lens and adjusting the beam condensing position by making the divergent lens reciprocate. According to the method, however, light entering the objective lens does not become parallel light. Consequently, it is difficult to narrow the beam to the condensing position, and a problem occurs such that measurement accuracy cannot be assured. In addition, since the divergent lens reciprocates, at the time of replacing the divergence lens for adjustment of the beam condensing position, a problem similar to that in the case of replacing the objective lens occurs.
Further, the displacement sensor of this kind is often used for in-line measurement in a factory or the like. During the measurement, a work is often changed. However, a lens in the sensor and a driving system cannot be replaced unless measurement is stopped for long time. It is also difficult for the user in the site to execute the replacement, so that it is difficult to employ the method in the site of performing in-line measurement.
On the other hand, when the beam condensing position is changed by adding a conversion lens to the coaxial confocal optical system, it is unnecessary to change the design of the sensor body, and it seems that the method also allows in-line measurement.
FIGS. 9 and 10 show an example of changing a working distance and a measuring range by attaching a lens holder 211 in which a conversion lens is assembled to a sensor head 210 in which the optical system of FIG. 8 is assembled. In the example of FIG. 9, a collimate lens 212 and a condenser lens 213 are assembled in the lens holder 211 to convert light condensed by the optical system in the sensor head 210 to once parallel rays and the parallel rays are condensed again. In an example of FIG. 10, light condensed and then expanded is again condensed by a single condenser lens 214.
In each of the diagrams, “a” and “b” show the working distance and the measuring range of the original optical system, and A and B express the working distance and the measuring range after the change.
As described above, theoretically, the working distance and the measuring range can be changed by adding a lens. However, when the direction of light once condensed is changed, light becomes susceptible to the influence of aberration of the lens and the like. As a result, the condensing range at the time of re-condensing the light cannot be sufficiently narrowed, and accuracy of measurement cannot be assured.
In the examples of FIGS. 9 and 10, the light condensed by the original optical system and expanded is incident on the lenses 212, 213, and 214 for correction. Consequently, when the height of the lenses 212, 213, and 214 for correction is changed, the working distance and the measuring range also fluctuate. In such a configuration, when the position of the lens holder 211 is adjusted in accordance with the height of the work W, measurement parameters also change, and a problem occurs such that adjustment cannot be performed easily.
In addition, in the method of FIG. 10, to condense diverging rays, the lens 214 having a large diameter is necessary, and it is necessary to set a distance D from the light condensing position by the original optical system to the lens 214 to be longer than the focal distance of the lens 214. It also causes a problem of increase in the size of the lens holder 211.
The present invention has been achieved by paying attention to the problems, and an object of the invention is to enable a working distance and a measuring range to be easily changed by adding a lens holder with a simple configuration and to assure measurement accuracy also after the change.